1. Field of the Invention
This invention pertains generally to non-contact thermal registration methods, and more particularly to high resolution and super-resolution non-contact optical methods for providing thermal sub micron characterization of active semiconductor devices.
2. Description of the Background Art
Advances are continually being made in semiconductor fabrication techniques that allow manufacturing smaller and faster semiconductor devices. It will be appreciated that these fast-switching devices also can generate an increasing amount of heat per unit area. Effective thermal management, therefore, is essential to achieve reliable operation of these semiconductor devices in the face of continuing advancement, and the characterization of thermal effects on the scale of these devices is becoming increasing important. It will be appreciated that achieving temperature measurements on increasingly smaller device geometries is non-trivial, particularly at spatial resolutions at or below one micron. Although a number of methods exist to measure the temperature on such a small scale, all current methods suffer from one or more shortcomings, such as lack of accuracy, measurement speed, the required sophistication of the experimental setup, and complexity of the required data analysis.
One application for thermal imaging is in relation to the use of Heterostructure Integrated Thermionic (HIT) coolers. By way of example, and not of limitation, this application will be described in relation to the current methods of thermal imaging so that the current methods may be more readily understood. Thermal management of small, hot semiconductor devices is often accomplished with the use of HIT micro coolers. These micro coolers can be integrated with other types of semiconductor devices to provide localized cooling. For example, an integrated HIT cooler with a semiconductor laser would provide for active temperature control to stabilize the wavelength of the laser, and thus improve reliability of wavelength division multiplexing (WDM) networks. In developing HIT coolers, it is important to experimentally gather measurements of cooling profiles for different device structures and geometries, such as for HIT micro coolers ranging in size from 10 μm to 100 μm. Currently, theoretical predictions for the cooler are higher than the performance measured so there is a need to identify and eliminate non-ideal effects.
Cooling as a function of the applied current to the cooler can be represented by the following quadratic equation:Cooling=αI−βI2 wherein a linear Peltier term αI is followed by a Joule heating term βI2, with alpha (α) and beta (β) being based on device parameters. Previous experimental measurements of HIT device cooling were achieved in the lab by using a micro thermocouple in contact with the device. The smallest conventional thermocouples with high accuracy that are available are approximately 50 μm in diameter. Using a microscope, the thermocouple was placed in contact with the device, the temperature was measured, and the output was monitored by a computer. Some typical room temperature results are shown in FIG. 1 along with the quadratic fit. The HIT coolers are shown to provide a maximum cooling of about one degree at about 180 mA.
Use of a thermocouple has a number of drawbacks which are primary result of its significant size in relation to the device under test, which in this case is a HIT cooler, such as in the 10 μm to 100 μm size range. Thermally coupled with the HIT cooler, or alternative circuits under test, the thermocouple represents a huge thermal load, and the heat required to warm up the thermocouple should be considered, as well as general heat losses or gains which may be conducted via the thermocouple to or from the device being tested. This thermal mass, generally being larger than the device itself, also extends the necessary measurement time scale as a consequence of thermocouple response time. Furthermore, the thermocouples require the careful use of thermal pastes and are susceptible to breakage. It will be appreciated, therefore, that a need exists to improve the method of thermal characterization.
There are various known methods of performing non-contact temperature measurements that possibly could be applied to the testing of HIT coolers, although such testing requires spatial resolution on the order of one micron and a thermal resolution which can preferably approach 0.1° K. It will be appreciated that a number of material properties are dependent on temperature that may be exploited for performing local temperature measurements.
For example, in liquid crystal thermography (LCT), a thin layer of liquid crystal is deposited onto a device under test. Traditionally the LCT method has used cholesteric crystals where the local color of the film is related to the temperature. An improved method uses the nematic-isotropic phase transition giving a spatial resolution of two microns and thermal resolution of 0.1° K. The phase transition causes a dark spot to be seen under a polarizing microscope. Since the phase transition only occurs at one temperature for a certain liquid crystal, the temperature registration only provides relative temperature measurement within a narrow temperature range. The stage temperature is set relative to the transition temperature, subsequent to which the surface temperature can be determined. Thus to generate a series of isothermal lines, the experiment must be repeated many times at differing stage temperatures.
Fluorescence microthermography utilizes a thin film of europium thenoyl-triuoroacetonate (EuTTA), having a florescence line at 612 nm, that is deposited on the surface of the sample and which is subsequently illuminated with UV light. Within a gap from 500 nm to 612 nm, there is no absorption and no fluorescence occur, and the quantum efficiency is therein a function of temperature. Thus, by measuring the intensity of the reflected light, and by applying an inverse transformation function, the surface temperature of the device can be deduced. Moderate complexity image processing is required to recover the relative temperature map.
Both fluorescence microthermography and liquid crystal thermography use the thermal properties of a well known substance that is deposited on top of the device, which should result in well calibrated results. However, the temperature of the layer is what is actually being measured, and the non-zero thermal resistance and the heat capacity of the deposited layer must also be considered. In addition, accuracy depends on considerations of layer thickness and possible non-uniformities.
Optical interferometry measures the thermal expansion of the material. This method looks at interference between a reference and reflected beam. The path difference of the rays can be measured very accurately by the interference pattern. The spatial resolution is limited by the size of the laser spot of the interfometer.
Polarization difference reflectometry utilizes a modulated incident beam with P and S polarization components that have different reflectivity components. By modulating the polarization of a laser probe, then taking the difference, the temperature of the sample can be determined from registering non-normal angles of reflection. This method requires active excitation of the sample, and as a result of the difference signal reaching a maximum at 88 degrees, wherein probe beam size must be considered.
Near field optical microscopy (NSOM) requires the fabrication of a fiber optic probe with an aperture of about 50 nm, such as may be created by stretching a fiber, coating the end in metal, and then etching to the desired profile. The probe is then placed close enough to the surface of the device and the near field can be transmitted through the fiber-optic, subject to a level of intrinsic loss. For a small aperture, the wave function of electromagnetic radiation is a decaying exponential; however, if the wave is not subject to excessive loss it will be transmitted down the probe. The technique is capable of providing spatial resolution which is higher than the diffraction limit of the radiation from the surface. The probe may be utilized to implement a number of measurement methods and is described in the literature for performing standard blackbody measurements, and for performing thermoreflectance probing. A few of the complications involved with the method are issues relating to the construction of the fiber optic probe, and its placement in sufficient proximity to the surface of the device to measure the near field. The probe should be positioned roughly half of the aperture size from the surface, to allow for near field detection. Typically, this requirement can be met by positioning the probe within 25 nm of the surface; however, any sudden contact with the surface can easily destroy the probe.
Infrared thermography makes use of the fact that objects or materials held at a temperature above absolute zero emit a level of infrared radiation. One class of objects, referred to as blackbodies, have an infrared radiation distribution which is well known. Classical infrared thermography utilizes Plank's blackbody law to determine the temperature of the object surface. By measuring the radiation intensity at a specific wavelength and making the blackbody assumption, the absolute temperature can be measured. The spatial resolution of the infrared image is determined by the diffraction limit. In reality, however, few objects can be considered “blackbodies” and Plank's law needs to be scaled for each object by a factor called the emissivity. The generation of accurate thermal images, therefore, requires a knowledge of the emissivity for each element within the image, which consequently complicates temperature calibration of the associated infrared camera images. Presently, high sensitive IR cameras operate at about a 3 μm wavelength.
In collaboration with Oak Ridge National Labs, an IR camera was utilized to measure the cooling on a 180×90 micron cooler. It was found, however, that even from a camera costing over one hundred thousand dollars, the image was not useful even on the largest coolers. The heat from the current probe was found to dominate the image and mask any effects of cooling at the surface. Furthermore the image was not normalized so as provide temperature profiling.
Thermoreflectance measurements have been utilized for registering low-resolution thermal profiles of devices. It will be recognized that the reflectance coefficient of a surface has a small linear dependence on temperature, wherein the change in reflection per unit temperature is called the thermoreflectance constant and is denoted by Cth. Thermal excitation of the surface may be provided by a heating laser, and the phase difference between the excitation pulse and the probe can be used to determine the thermal wave propagation velocity of the solid, or the thermal resistance of a material, at the surface. The semiconductor device itself has also been utilized as the excitation source and the thermal change in the sample can thereafter be determined. Researchers have experimented with metal interconnects which excite a metal trace with a current pulse and register the change in reflection. The reflection changes are calibrated with thermistor measurements. Other researchers have experimented with measuring thermoreflectance when heating a 35 micron MOS transistor, and temperatures in semiconductor lasers. However, in all these instances the small thermoreflectance coefficient results in the generation of a measurement having a low signal-to-noise ratio, and the technique is unable to provide absolute measurements. These direct thermoreflectance measurements are capable of providing a resolution on the order of 10° K.
Therefore, it will be appreciated current methods for performing thermal measurements on small circuits and devices are difficult to construct and calibrate, while they often provide insufficient spatial and thermal resolution. The present invention overcomes those deficiencies.